The detection and identification of low contrast objects on or below the surface of the ocean has a multitude of commercial applications from detecting dolphin free schools of tuna to counting salmon on the rivers and estuaries of the northwest. Additional uses include the location of debris from maritime and aircraft accidents and the possible location of maritime survivors in a search and rescue operation. Military applications obviously include the detection and location of subsurface threats such as mines, and the avoidance of navigational hazards, such as reefs and shoals. Recent applications in the area of homeland security include the monitoring of harbors, rivers, and lakes for towed contraband and subsurface intrusion by divers.
Optimum processing of ocean imagery for this purpose requires spatial and temporal registration of the image sensor over the entire imaged field of view. Spatial or temporal image mis-registration decorrelates the images and results in a loss of processing gain. Any loss of processing gain results in the reduced ability to detect low contrast targets in the high background imagery of the ocean's surface. Processing gains of at least 33 dB are possible for a two-color multispectral image. The processing three or more colors can yield an additional 6 dB of gain.
Prior techniques include the use of multiple cameras, single cameras with multiple sensor arrays, and single cameras with a single sensor array. Prior single cameras such as camcorders or digital cameras with single sensor arrays impart both spatial and temporal image mis-registration since the color mosaic (FIG. 1A) is affixed to the sensor. A color ‘pixel’ is comprised of the 4 subpixels 102, 104, and 106 shown within the dotted lines. The subpixels do not image the same points on the water; hence give rise to spatial misregistration. A spatial mis-registration of one sub-pixel has been shown to produce a 10 dB loss of processing gain. If the sub-pixels are not sampled simultaneously, a temporal mis-registration will result. A difference in the temporal sampling of the pixels imparts a loss of correlation between the channels. For every factor of ten in the temporal difference (10 milliseconds instead of 1 millisecond) the correlation will lose a factor of ten (0.9 instead of 0.99) and the processing gain will lose 20 dB, with a resultant reduction in the object detection depth capability.
A single camera with multiple sensors (FIG. 1B) imparts the mis-registration at the time the sensors are affixed to a beam splitter or prism. As shown in FIG. 1B, the three light components 120, 122, and 124 are spectrally separated into red (122), green (120), and blue (124) through the use of dichroic filters 110 and 112, and prisms 114, 116 and 118. Although dichroic filters can be manufactured with minimal polarization, the multiple reflections in prisms 116 and 118 create a light output that is polarized, primarily in the green and blue channels, while the red channel is relatively un-polarized. This difference can result in a loss of processing gain of up to 9 dB if used for the present application.
The use of two cameras requires that both lenses have the same magnification, f-number and focus. Furthermore, some mechanical means of aligning the images to register the same image plane is required in pitch, roll, and yaw. In general, due to coma and other lens effects, this can only be accomplished over 80% of the image for arrays with pixels on the order of 50 or more microns. Alignment to 4 microns is difficult to attain and a re-alignment must be done on a daily basis. Modern sensors have pixels on the order of 9 microns, so it becomes virtually impossible to maintain image registration to less than ½ of a pixel without a subsequent loss of processing gain of 5 dB.
Images of the ocean's surface contain several clutter components; light scatter in the atmosphere between the camera and the ocean surface, glint or light reflection from the water surface, hemispherical light scatter from the atmosphere above the ocean, and light from the upwelling irradiance of the water with and without an object.
Consequently there is a need for a multispectral imaging system that has both spatial and temporal registration over the entire imaged field of view as well as being able to reduce or eliminate atmospheric and hemispheric light scatter, light reflection from the waters surface as well as upwelling irradiation from the water.